Among the various applications in meteorology and hydrology the mitigation of the effects of severe weather is one of the primary aims of operational hydrometeorological services. The WMO has therefore established relevant programmes long before the start of the IDNDR: the Tropical Cyclone Programme, Environmental Emergency Response and the Severe Weather Warnings which gained new momentum during the last decade. The expanding population and related socio-economic developments have increased the need for timely prediction and mitigation of such disasters and so has the need for suitable infrastructures and co-operative measures. Although the overall number of disastrous events has not significantly increased, their damaging effects on the economy appears to be growing. In addition to the short-lived phenomena, the long-term aspects of weather and climate are now becoming increasingly important. Desertification, water resource management, climate change and their impact are key issues and have stressed the need for the WWW as a reliable system to support relevant disaster mitigation programmes on a global, regional and national scale.